Over the past ten years, molded silicone rubber has been used in various automotive sealing applications such as valve rocker cover and oil pan gaskets. The reason for this development is to be found in the outstanding properties of silicones. As is well-known, silicones have high temperature stability and cold temperature flexibility in conjunction with low stress relaxation and ease of fabrication. Heat cured silicone rubber compounds have a proven track record in engine sealing applications and have provided the longevity of service required by the automotive community.
Recently the automotive manufacturers have experienced some problems in oil permeability, known as "weepage". Silicone rubber gaskets have a propensity to weep motor oil through the rubber, resulting in accumulation of oil on the outside surface of the engine. Although this does not result in gross leakage, it causes an unsightly slick which has become a significant component of engine warranty costs.
One approach which has been shown to moderately reduce weepage in methyl vinyl silicone compounds is the incorporation of relatively large amounts of extending fillers. Unfortunately this adversely impacts the physical properties and renders gaskets molded from these materials unsuitable for use in cam cover sealing applications which use plastic composite covers. This is due to unacceptable increase in compressive modulus which causes the covers to crack under load. In addition, the reduction in weepage is only partial which does not satisfactorily resolve the problem.
Another approach is to use gaskets made from fluorosilicone elastomers. It has been shown by the major silicone suppliers and gasket fabricators that fluorosilicone elastomers as a special category of silicones do not exhibit oil weepage. However, gaskets from fluorosilicone elastomers are more expensive to fabricate than gaskets from silicone gaskets. Further since fluorosilicones are halogenated elastomers, disposal of the fluorosilicone gaskets may give rise to environmental concerns. Thus there is a need for automobile gaskets which minimizes the fluorosilicone content but offers substantial reduction to oil weepage.